We have developed specific inhibitors of, and substrates for, deubiquitinating enzymes (DUBs) acting upon ubiquitin, NeddS, SUMO-1, SUMO-2/3, and ISG15 (collectively referred to as ubiquitin-like or UBL proteins). We refer to these as the UBL-X panel of reagents. The specificity of these reagents is outstanding and methods to assess cross reactivity between DUB homologs and UBL paralogs have been developed. These reagents have allowed us to define the likely specificity of a number of individual DUBs, led to some surprising conclusions regarding the spectrum of activities exhibited by different classes of DUBs, and suggested new opportunities for developing and characterizing inhibitors that specifically interfere with selected pathways of UBL protein metabolism. In the coming period: We will use the UBL-X panel of reagents in quantitative labeling and kinetic approaches to determine the amounts and paralog specificity of active desumoylating enzymes in lysates from control and stressed cells. The specificity will be compared to that of the purified catalytic domains. We will test the hypothesis that changes in cellular levels of sumoylated proteins in response to cellular stress are controlled by levels of active desumoylating enzymes. We will determine the enzymatic specificity of the DUB activity exhibited by the SARS virus papain-like protease using the panel of UBL-X reagents. This information will suggest possible cellular substrates and be used to mount a high throughput drug screen to identify antiviral agents that may be useful in limiting the spread of coronavirus infections. We will use the panel of UBL-X reagents to evaluate the specificity of inhibitors that have been reported to inhibit deubiquitinating enzymes. This will provide a rapid and selective method to evaluate the global effects of "specific" inhibitors on the spectrum of deubiquitinating enzymes. In the broadest sense, these studies will contribute to our understanding of UBL protein metabolism and guide development of inhibitors and drugs for the treatment of cancers and microbial infections.